1. Field of the Invention
This invention relates to a vehicle speed control system, more particularly to a vehicle speed control system equipped with a cruise controller and a CVT (Continuously Variable Transmission).
2. Description of the Related Art
Japanese Laid-open Patent Application No. Hei 10 (1998)-29448 teaches a cruise controller for a vehicle equipped with a CVT. This cruise controller has a followup operation to follow up another vehicle running ahead of the subject vehicle. In this prior art system, when the followup operation is in progress, the ratio (speed ratio) of the CVT is controlled at a fixed ratio and it is determined whether required torque is available. If the required torque is not available, the air/fuel ratio to be supplied to the engine is enriched, thereby reducing the engine speed fluctuation and preventing wandering of the engine speed from happening.
Generally speaking, in the cruise controller including that mentioned above, when the vehicle operator manipulates the set switch to input the intention to cruise, the controller effects an initial operation (initialization) for operating a throttle actuator (for moving the throttle valve) from its initial position (corresponding to the fully-closed throttle position or thereabout) to a position corresponding to the throttle valve opening or a position at which the vehicle operator inputted the instruction to cruise control. Depending on when the vehicle operator releases the accelerator pedal, the throttle actuator may not reach the required position or in the worst case, the throttle valve may remain at the fully closed position.
This will be explained with reference to FIG. 14. Assume, for example, that during uphill driving the vehicle operator expresses a desire to cruise control by turning the set switch on at time point t 1. The initial operation begins from this time point.
Normally, the vehicle operator would release the accelerator pedal simultaneously with turning on the set switch at time point t1 . However, it may sometimes happen that the vehicle operator continues to press down on the accelerator pedal even after time point t1. In such a case, the vehicle speed will exceed the desired speed instructed through the set switch and the cruise controller misinterpreting this to mean that the throttle actuator was moved too far in the opening direction, will move the throttle actuator in the closing direction at time t2, for example, in order to lower the vehicle speed.
Assuming that the vehicle operator releases the accelerator pedal at time point t3, the throttle valve will immediately be shut to the fully-closed position or thereabout. As mentioned below, the shift controller controls the ratio (speed ratio) of the CVT by retrieving it from predefined characteristics (shift scheduling map) using the throttle opening and the vehicle speed as address data. The shift controller would therefore respond to the closing of the throttle valve by controlling the ratio to the high side at time point t3.
As a result, torque will fall and the vehicle speed will drop markedly. In response to the lowering of the vehicle speed, the cruise controller will move the throttle actuator in the throttle opening direction in order to raise the vehicle speed toward the desired speed. With this, the shift controller will control the speed ratio to the low side. However, since the throttle valve has been driven farther than necessary in the opening direction, the shift controller will further control the speed ratio to the low side. More torque than needed will therefore be produced.
When the vehicle speed approaches the desired speed, the cruise controller will restore the throttle valve in the closing direction. Since the speed ratio was controlled unnecessarily far toward the low side, the vehicle speed will overshoot the desired speed. In order to correct this, the cruise controller will control the throttle valve in the closing direction, causing the vehicle speed to undershoot, and in response to this, once again control it in the opening direction, causing torque to become insufficient.
These repeated ups and downs in the vehicle speed makes the vehicle operator feel uneasy and, at worst, may lead to control hunting. Although not illustrated, a similar situation may also arise when the vehicle operator releases the accelerator pedal too quickly. One of the causes for this inconvenience lies in the high-precision speed ratio control capability of the CVT.
Although the earlier prior art mentioned above teaches a cruise controller for vehicles equipped with a CVT, it does not go beyond simple measures such as fixing the speed ratio when the followup operation is in progress and is therefore incapable of offering a solution to the issues discussed above.
An object of this invention is to eliminate the drawbacks of the prior art by providing a vehicle speed control system that, when applied to a vehicle equipped with a CVT connected to the vehicle engine for transmitting the engine output torque while continuously varying the speed ratio of the CVT, can reduce throttle opening change when cruise control is effected so as to control the vehicle speed toward a desired speed in response to the instruction to cruise control inputted by the vehicle operator, thereby decreasing vehicle speed fluctuation.
For realizing this object, the present invention provides a system for controlling a speed of a vehicle having a continuously variable transmission which transmits output torque generated by an internal combustion engine mounted on the vehicle to drive wheels of the vehicle, and a cruise controller which controls the speed of the vehicle to a desired speed in response to an instruction to cruise control inputted by a vehicle operator, comprising: operating condition detecting means for detecting operating conditions of the engine and the vehicle including at least an opening of a throttle valve and the speed of the vehicle; a throttle actuator which is connected to the throttle valve to move the throttle valve; initialization conducting means for conducting initialization to drive the throttle actuator from an initial position to a position corresponding to the detected opening of the throttle valve at which the instruction to cruise control is inputted; and shift controlling means for controlling a speed ratio of the continuously variable transmission based at least on the detected opening of the throttle valve and the detected speed of the vehicle; wherein the shift controlling means controlling the speed ratio of the continuously variable transmission to a value, at which the instruction to cruise control is inputted, for a predetermined period of time since inputting of the instruction to cruise control.